The present invention relates to radiochromic imaging methods for generating a permanent, colored two or three dimensional spatial representation of an irradiation pattern wherein the intensity of the color of the image correlates to the dose level of irradiation, and more particularly to methods of utilizing these imaged color changes in applications such as tissue-equivalent dosimeters for medical radiotherapy to assess precise radiation targeting within a patient and in-other applications such as chromatographic analysis to determine the location and quantity of separated components.
Quantification of objects in a given environment has been a longstanding requirement since man has been able to count. Beside the obvious need in commerce to know how many of one item might be traded for another, there are many other areas where quantification plays an important role. For example, in the physical sciences, quantification of exposure to chemicals or radiation, or dosage of medicine all play a role in our well being. Even when no direct contact with these materials is an issue, quantification plays an important role in analytical testing for these materials.
Quantification can be performed directly (i.e., by counting the objects of interest) or indirectly (i.e., by correlating the number of objects to some other measurable parameter).
Indirect quantification has been widely used in correlating the intensity of color to the quantity of a substance. In spectroscopy the relationship is defined by Beers Law. Simply stated, there is typically a correlation between the amount of a given material and the intensity of light absorbed by that material. If the absorbed light is of a given wavelength or range of wavelengths within the visible spectrum then the absorption of this light creates color.
It has also been known that the interaction of electromagnetic radiation and matter can create new chemical materials. In some cases the new material will be colored. It is then possible to correlate the three parameters: amount of radiation, amount of reactant matter, and intensity of coloration of the product. In this manner if two of the three parameters are known then the third can be quantified indirectly. Studies of this kind can be conducted either over time or space to obtain a temporal and/or spatial quantification of either the amount of radiation or the amount of the reactant material if the intensity of the coloration can be determined.
Chromatographic techniques for separating multicomponent mixtures has been an invaluable tool to the scientific community. Such techniques provide the ability to not only separate but also quantify and ultimately identify the various components in the mixture. Depending on the specific chromatographic technique many different types of materials can be separated from one another even though they may be structurally very similar. Such materials include anionic and cationic ions, simple organic molecules and more complex organic polymers. These polymers may be synthetically derived as is the case with polyesters, polyethers, or polyolefins or may be naturally produced as are proteins, polynucleotides, or complex starches and carbohydrates. Separation methods can be based on chemical or physical properties such as electronic charge, chemical reactivity, molecular mass, or molecular volume.
In virtually all cases, the mixture to be separated must be incorporated in a system that has a mobile phase and a stationary phase. The mobile phase carries the individual components across the stationary phase and may consist of gas, a liquid, or mixtures thereof. Most commonly inert gases such as nitrogen, helium, carbon dioxide or the like are used. Common liquids include pure water, salt-containing water, organic solvents and mixtures thereof.
The stationary phase may be composed of materials that are either inertized or activated and may be derived from synthetic or natural materials. The critical factors in selecting a stationary phase are that it must be essentially stable over time and not be soluble in the mobile phase. Another important attribute of the stationary phase is that it provide sufficient adsorption of the individual components to effect separation. This is typically achieved by having the stationary phase comprise very small particles having a large surface area or in some cases to have the stationary phase have hollow portions that the individual components must migrate.
Many materials can be employed for use as a stationary phase including insoluble salts, clays, talc, polyolefins, resins, and the like. The interaction of the individual components with the mobile phase and the stationary phase causes the individual components to migrate through the stationary phase at different rates, thereby causing them to separate. Separation is thus obtained by each component having a unique rate of migration. In a fairly recent innovation the stationary phase and the mobile phase can actually be reduced to one phase. In this case the stationary phase is in the form of a polymeric material dissolved in the mobile phase to essentially form a gel material. In the gel the stationary phase does not migrate, especially if the stationary phase polymeric material is crosslinked. In this system there is an extremely high propensity for interaction of the individual components with the stationary phase since contact is essentially made with individual molecules of the polymeric stationary phase. Bulk effects that would limit interaction of the individual components and the stationary phase are essentially eliminated.
The flow of the mixture to be separated is achieved by having the mobile phase flow either under gravity or pressure or by diffusion from an inlet side to an outlet side. More recently a technique that does not require the mobile phase to physically flow has been achieved. In this method the mixture is dissolved in the mobile phase and the stationary phase is placed in a environment that subjects the system to an electric current and the components in the mixture are separated due to their electrical charge and the interaction with the stationary phase. One such method is electrophoresis, wherein substances disposed within a gel are subjected to an electrical current. These substances migrate in the gel, under the influence of an electrical field.
Unless the components in the mixture are inherently colored, visualization of the separated components requires an additional step. This can be achieved without the need of other materials by such techniques as measuring spectroscopic absorbtivity in the ultraviolet or infrared region of the components themselves. In other techniques color formers are applied to the migrated components to generate color regions. Coloring the substances with a dye reveals their migrated position within the gel. This electrophoretic method is used in present day DNA and protein analyses. A problem common to DNA testing, or electrophoretic methods, is the overlap of certain migrating substances within the gel. Those overlapping substances cannot be easily distinguished from each other. Other problems that typically arise when employing chromatographic techniques is that even when coloring has successfully identified the location of the various components, the color fades with time. This is of particular concern if the object is to quantify the amount of each component using the intensity of the color to correlate to the quantity of the component.
The quantification of radiation exposure has been performed directly using such tools as Geiger counters. Techniques of this type provide basically bulk information and cannot be used for determining the irradiation level at various depths within a subject material. In other words a three dimensional, spatial profile of irradiation dose is not feasible with such direct techniques. This type of information, however, is invaluable in such areas of science as radiation therapy and food sterilization.
It is critical in medical radiotherapy that the exact radiated area in the body, or exact direction of a radiation beam, with respect to a patient is known. Patients being irradiated with beams of high dosages of radiation can be severely harmed if the radiation direction or its ultimate direction is not precisely known. For example, a tumor located near a sensitive body organ, such as a spinal cord, must be precisely irradiated. Any radiation misalignment can cause damage to the spinal cord, with the unintended result of severely and permanently injuring the patient. The simulation gel of the present invention is irradiated to determine the precise extent of the volumetric distribution of radiation which the beam will target.
Recent advances in computer and radiation delivery techniques have increased the need to precisely define the radiation target volume. Radiotherapy methods are now more sophisticated, resulting in a need for more detailed information about the typically more complex radiation dose distribution than was previously the case. These treatment modalities include high dose-rate, or highly directed types such as conformal (CRT), intensity-modulated (IMRT), brachytherapy, pencil-beam, and stereotactic radiotherapies.
The common aim of these new methods is to deliver relatively high radiation doses to malignancies, within narrowly prescribed physical margins to spare surrounding normal tissues. In order to achieve beneficial results without increasing the morbidity rate, an extraordinary investment in terms of professional-time and effort must be made in the quality assurance and treatment planning processes. These processes could be facilitated by new techniques for three-dimensional radiation dosimetry.
Several years ago a tissue-equivalent system was developed for radiation dose distribution studies, that was capable of being imaged by optical scanning. The system consisted of an aqueous agarose gel (1%) that contained ferrous ions and xylenol orange. Radiation-chemical reactions initiated in the water component disposed in the gel, caused a chain reaction oxidation of 2-valent ferrous ions, which were converted to 3-valent ferric ions. The reaction was contained in local concentrations proportional to the local radiation dose deposited. It was already established that the ferric ion distribution could be imaged by MRI techniques, as a consequence of the paramagnetic nature of the ferric ions. The presence of xylenol orange allowed the rapid formation of a colored complex between ferricions and xylenol orange. This provided the possibility of using three-dimensional imaging via optical scanning techniques. This technique was potentially much more cost-effective and convenient than MRI.
More recently, a simple optical scanning method was developed. The newer method used a CCD camera for three dimensional imaging of the dose distributions of the radiated ferrous/xylenol orange gel. Advantages of such an aqueous-based system like this, was its ease of preparation, and non-toxicity.
A major disadvantage of the aqueous ferrous/xylenol orange system, was the quickly degrading image that resulted from the diffusion of the colored radiation-induced complex. It was, therefore, necessary to accelerate the scanning procedure. This was problematic when low dose rates and hence, long irradiation times were needed. It was also difficult with small sized irradiated fields, where small amounts of diffusion represent large proportional losses of image coherence.
Another prior art three-dimensional dosimetry system is one based on radiation-induced polymerization of acrylamide gels. The gels turn from transparent to milky white following exposure to radiation, and were scanned optically. The images did not degrade by diffusion. This system had the disadvantages of greater complexity of preparation (oxygen had to be rigorously excluded). Toxicity of the acrylamide monomer also posed concomitant disposal problems. The attenuation was dependent upon scattering, rather than by absorption, of the radiation, thus limiting the sensitivity.
It is one object of the method of this invention, to provide a color forming reactant that on exposure to radiation generates a color.
It is yet another object of the invention to provide a source of radiation that causes a color forming reactant to generate a colored product.
It is another object of the method to incorporate a color forming reactant into a matrix or stationary phase.
It is still another object of the invention to provide a process for forming a colored image that is insensitive to the presence of oxygen.
It is yet another object of the method for the colored product formed by irradiating the color forming reactant in a matrix of elements, constituting a defined shape and volume, to spatially quantify the amount of radiation that is absorbed within each element of the matrix.
It is still another object of the invention to provide a tissue-equivalent dosimeter that provides a stable image that is directly related to radiation dose.
It is yet another object of the method, that a radioactive label is tagged into individual members a chemical mixture. The mixture is subjected to a chromatographic system having a color forming reactant uniformly present in the matrix such that after separation the radiolabel tag will generate color at the migration site. The level of radiation will be used to quantify the amount of each individual component.
In accordance with the present invention, a system has been developed that provides a stable color representation from the interaction of radiation and a color forming agent. The color representation in a given spatial area or volume accurately correlates to the radiation dosage impinging on the color forming reactant in the spatial area or volume of the matrix.
In one series of embodiments the radiation is supplied by an external source and exposes the color forming reactant present in either a two or a three dimensional dosimeter matrix. The quantification of optical density within the respective units of pixels or voxels of the matrix accurately correlate to their level of radiation absorption.
In a second series of embodiments the radiation is supplied by radiolabel tagging the individual components in a mixture and having the color forming reactant uniformly disposed throughout a stationary substrate. Subsequently, a chromatographic technique is performed to separate the components, allowing the radioactive label in each migrated component to cause color formation in the area of migration.
The current invention reflects the discovery that better quantification can be obtained in electrophoretic methods and radiation testing with the use of specific tetrazolium salts. Tetrazolium salts have been long used as biological indicators. There is considerable literature on the use of tetrazolium compounds as radiochromic dosimeters.
In aqueous solution, these soluble compounds are reduced by radiation to highly colored, insoluble compounds, known as formazans. There are at least two hundred and forty different tetrazolium compounds, and four hundred and fourteen formazans. An example of a tetrazolium compound that can be used as a radiochromic dosimeter is nitro blue tetrazolium (NBTZ). This compound has the following structure: 
Reduction of NBTZ leads to the colored formazan, depicted below: 
A necessary characteristic of the color indicators of this invention, is their conversion from soluble to insoluble substances in water along with the color change. As aforementioned, one of the unique characteristics of the above tetrazolium compound is that the resulting formazan molecule, produced by radiation, changes both its color and solubility. In an aqueous gel, the formazan would not diffuse freely through the aqueous medium as do soluble materials such as the ferric/xylenol orange complex. Instead it attaches to surfaces, such as the surface of the gel structure. This forms the basis of a non-diffusing image of the radiation dose in three dimensions, in an aqueous non-toxic medium.
It is an aspect of this invention to provide an improved method of producing a radiochromatic change within a gelatinous material.
It is another aspect of the invention to provide a radiochromatic method whereby a precise location of a substance disposed within a gel can be determined.
It is a further aspect of this invention to provide a radiochromatic method whereby the migration of substances within a gelatinous medium can be precisely determined.